• For semelparous organism where tradeoff between m and p(g) are represented by concave Curves, RE will be all or none.
• For iteroparous organism where tradeoff between m and p(g) are concave RE will be intermediate, except at last age of reproduction. As residual reproductive value (RRV) decreases the optimum reproductive effort will increase.

If l(x) and m(x) are affected by environmental variability a "bet-hedging’ strategy may evolve.

Bet-hedging evolves when there is a trade-off between mean and variance, where phenotypes with reduced mean fitness may be at a selective advantage when there is variance in juvenile or adult survivorship.

For Ex.

Idea: B represents the ‘bet-hedging’ strategy – less energy expended to offspring in good years even though more is possible. Having fewer offspring in good years enables more to be produced in bad years.

When the fitness of a genotype varies over a generation it is appropriate to measure its contribution in terms of the "geometric mean" fitness rather than its "arithemetric mean" fitness.

Why? Because population increase occurs geometrically not linearly.

So, from our example let’s look at the difference.

Where,

Genotype:

Empirical evidence:

Reid (1987) studied population of sea gulls where natural range of clutch sizes is 1-3 chicks, and he artificially altered broods to range from 1-7 chicks.

Found that adult survivorship declined significantly when > 3 chicks, however, fledgling success was disporportionately higher in large broods (seems to have a selected advantage. These leads to higher adult fitness in good years but in poor years few broods survive and adult mortality is high. So, smaller brood size, on average, results in higher fitness.

Over the past few lectures we have examined question that attempt to explain the patterns of life history traits such as:

Why do some organisms breed only once?

Why do some breed early and others late?

Why few large offspring vs. many small offspring?

Two major parameters - RE and Iy

semelparity

small body size

iteroparity

larger body size

1. stable environment
2. contest competition
3. dispersal not necessary (low)
4. size-dependent survivorship

result: small clutch size

large young

r low

1. total energy available to parent
2. amount of RE available (given trade-off between current and future reproduction.
3. Amount of Iy available

1. unstable environment
2. exploitative competition
3. dispersal important
4. size-neutral

These predictions have been brought together to form a concept that has been widely applied to explain life-history patterns, referred at the r and K selection paradigm. The letters refer to parameters of the logistic equation where:

• r-selected individuals have been favored to reproduce rapidly.
• K-selected individuals have been favored to for their ability to make a large proportional contribution to a population which remains at carrying capacity.

Under K conditions, life-history characters are influenced by density-dependent conditions.

• Const., seasonally-predictable (low random envir. fluctuations), but not to say that the envir may be harsh.
• consequently, predicts crowded population, constant.
• competition intense among adults
• young compete for limited resources
• size is important

In other words: low RE, high Iy

Under r conditions, life-history influenced by density-indep cond, habitat unpredictable, intermittently pop exp. benign periods of rapid growth free from competition – other times unavoidable mortality (unpredictable).

Consequences – periods when high r is favored, mortality independent of size of offspring and populations density, survivorship dependent on environmental conditions.

In other words: RE high, Iy low

Originally concept applied to contrast species that were good at rapidly colonizing ‘empty’ islands versus species that were at maintaining themselves on islands once many colonizers had reached the island.

Are low RE, high Iy and vice versa really correlated?

Higher vertebrates – large size, long-life, low reproductive output (homeostatic).

Insects – small size, short-lived, high reproductive output (not homeostatic)

Ecological approach to studying the environmental basis for life-history variation. Study two or more populations (within species best, or at least closely-related species).

Examined the influence of density and density-independent affects on life history traits in dandelion.

Experimentally introduced a regime of disturbances in three habitats:

High D.I. mortality

1-mowed every week

2- mowed every 3 mos.

High D.D. mortality

3- mowed once/year

3 biotypes (electrophoretically distinct morphs)

Biotype 1: r-type favored (exploitative competition). Vs Biotype 3: K-type favored (contest competition)

Other study: Reznick and Endler - examined impact of predation on life history in guppies

dry wt. embryos

dry wt. body & embryo

More recent work by Reznick and Endler show that individuals tranplanted across habitats show genetic changes over time that conform to ecological condition - confirmation of evolution of characters direct result of ecological conditions.

Components that describe life history traits:

Reproductive effort (RE)

Investment of effort (Iy)

These parameters have been linked together in the concept know as r and K selection Where,

and

Wylber et al. 1970

Scleropus olivaceous Lizard

Juvenile mortality is high but indep. of offspring size - source of mortality is predation not competition.

Chelonia mydas (Green sea turtle)

3- Physiological demands - low RE when RE should be high

e.g. reduced RE may be a result of lower total energy available.

e.g. Freshwater vs. brackish water populations of mosquitos. Total energy available is lower in brackish water populations due to higher osmoregulatory demands.

4- Phylogenetic constraints

e.g. Albatross - brood patch only able to accommodate single egg. To commit higher RE, requires concomitant morphological change.

Proximate explanation.

Many life-history studies report